1. Field of the Invention
This invention relates to an exposure method for conducting a process on a solid substrate using exposure, a process for sequential extension and/or preparation of a nucleic acid or peptide using the exposure method and an exposure device for conducting the exposure method.
2. Related Background Art
Recently, so-called sequencing by hybridization (SBH), in which hybridization of multiple nucleic acid probes is used to determine a base sequence in a particular target nucleic acid, has been intensely investigated. For example, U.S. Pat. No. 5,202,231 has disclosed the principle of SBH. EP Patent Publication 0373203 B1 has described an SBH method and a device using a solid-phase DNA array. Furthermore, U.S. Pat. No. 5,445,934 has described a stepwise process for preparing a nucleic acid combining a photolytic protective group and photolithography as a method for forming a two-dimensional DNA array on a solid-phase substrate.
Chemical synthesis of a nucleic acid has recently become a popular technique, and a DNA synthesizer has been marketed from several companies. There are several known methods for preparing a nucleic acid, among which a phosphoramidite method is most typical. The phosphoramidite method sequentially extends/synthesizes nucleotides at 5xe2x80x2 terminal side starting from a nucleotide with desired bases bound to a solid-phase substrate such as glass beads at its 3xe2x80x2 terminal. In the process, hydroxy group at the 5xe2x80x2 terminal of a synthesized nucleic acid strand is protected with a protective group such as dimethoxytrityl, which is then removed lust before reaction for introducing a next nucleotide. Generally, this protective group is removed under the acidic condition. The above process in U.S. Pat. No. 5,445,934 is basically an application of the sequential synthesis of a nucleic acid on a solid-phase substrate, but the above method for removing a protective group under the acidic condition is not suitable for synthesizing multiple different nucleic acid strands in a two-dimensional array because one range (matrix) forming a two-dimensional array has a size of several ten xcexcm to several hundred xcexcm and the number of matrices, i.e., types of nucleic acid strands, is several hundreds to several millions. It is basically difficult to individually place these many small regions under the acidic condition for deprotection. Thus, the above U.S. Pat. No. 5,445,934 uses a substrate to which a photolytic protective group (strictly speaking herein, a functional group) is attached via a linker as a starting material. First, a protective group at a position to which the first base (one of A, T, G and C) is to be introduced is removed according to the principle of photolithography, i.e., by partial exposure using a photomask to introduce a desired nucleotide, where hydroxyl group at 5xe2x80x2 terminal in the first nucleotide is similarly protected by a photolytic protective group. This procedure is repeated four times to introduce all the first nucleotides. Then, the second nucleotides are similarly introduced. The above procedure is repeated 4xc3x97N times (N is the length of the nucleic acid strand) in total to provide a nucleic acid strand with a desired length. The number of the type of prepared nucleic acid and the overall size of the formed DNA array depend on the size of the substrate used and the pattern of the photomask used, basically allowing us to prepare a two-dimensional DNA array with a similar fineness to that achieved in a semiconductor process. U.S. Pat. Nos. 5,445,934 and 5,424,186 have described in detail a photolytic protective group which may be used in such a process.
However, preparation of a two-dimensional DNA array using the above photolytic protective group may require 32 to 40 pieces of photomasks even for synthesizing an octamer to decamer nucleic acid which is a minimum requirement as a nucleic acid probe, and the corresponding number of exposure operations and associated steps. Furthermore, for preparing 18-mer to 20-mer nucleic acid representing an adequate length as a DNA probe, 72 to 80 pieces of photomasks are required and steps and a time taken for the process are enormous. Since a photomask is basically a consumable material, increase in the number of photomasks leads to significant increase in a cost.
In view of the above problems, an objective of this invention is to provide a pattern exposure process and a device which can be used for effectively preparing a DNA array or peptide array with a lower cost.
Another objective of this invention is to provide a method for effectively forming a nucleic acid array and a peptide array.
An embodiment of a method for pattern exposure which can achieve the above objectives is a method for pattern exposure comprising the step of exposing a photosensitive material on a solid-phase substrate by irradiating it with a light as a pattern, wherein the photosensitive material is selectively exposed with beams selectively emitted from multiple vertical cavity surface emitting laser (VCSEL) aligned as an array.
A method for preparing a nucleic acid array which can achieve the above objectives is a method for forming a nucleic acid array where multiple nucleic acid strands attach to multiple positions on a substrate surface and each of the nucleic acid strands has an inherent base sequence, comprising the steps of:
i) preparing a substrate with the surface on which a nucleic acid is bound at multiple positions, in which one end is bound to the substrate while the other end is protected with a photolytic protective group; and
ii) selectively irradiating given nucleic acids among the multiple nucleic acids bound to the substrate surface with a light using an exposure device where multiple VCSEL sources are aligned as an array such that each of the photolytic protective group may be irradiated with each beam from the light source to remove the photolytic protective group and then the nucleic acid strand is extended by binding a given nucleotide according to the base sequence.
A method for preparing a peptide array which can achieve the above objectives is a method for forming a peptide array where multiple peptide strands attach to multiple positions on a substrate surface and each of the peptide strands has an inherent sequence, comprising the steps of:
i) preparing a substrate with the surface on which a peptide is bound at multiple positions, in which one end is bound to the substrate while the other end is protected with a photolytic protective group; and
ii) selectively irradiating given peptides among the multiple peptides bound to the substrate surface with a light using an exposure device where multiple VCSEL sources are aligned as an array such that each of the photolytic protective group may be irradiated with each emission beam from the light source to remove the photolytic protective group and then the peptide strand is extended by binding a given peptide according to the sequence.
An embodiment of an exposure device which can achieve the above objectives is an exposure device for exposing a photosensitive material on a solid-phase substrate to a patterned beam, wherein multiple VCSEL sources are aligned as an array, and having a configuration such that the photosensitive material can be selectively irradiated with each emission beam from the light source.
This invention employs a VCSEL as a light source for exposure in photolithography for solving the above problems in the prior art. Specifically, in a process requiring exposure on a solid-phase substrate, exposure is conducted by emission, lighting or light showing (hereinafter, collectively referred to as xe2x80x9cemissionxe2x80x9d) of each of the multiple VCSEL aligned as a two-dimensional array in a desired pattern by a given procedure. Using such a method, since an exposure pattern may be varied by changing an emission pattern of the surface emitting lasers aligned on the two-dimensional array, necessity of individually changing a photomask for each pattern may be eliminated. In other words, it is not necessary to prepare photomasks corresponding to individual patterns or to change a photomask, resulting in easy exposure operation. Furthermore, since a photomask as a consumable material is not needed, an expense for exposure and a running cost may be reduced. On the other hand, compared with scanning exposure with a laser beam, since the material is exposed as a desired pattern in one time, it may basically eliminate a time for scanning. Furthermore, since a surface emitting laser basically provides a stronger optical power per an area, a time for exposure may also reduced. Additionally, there is another advantage that a program for pattern exposure is considerably simpler and more convenient than that required in laser-beam scanning.